An Electrically Erasable Programmable Read Only Memory (EEPROM) is a type of nonvolatile memory that is reprogrammable. EEPROM cells are arranged in an array and located at the intersections of rows and columns. An EEPROM cell is generally comprised of a floating gate transistor and a select transistor. The floating-gate transistor can be programmed by grounding the control electrode and raising the drain voltage. Tunneling causes electrons to be transferred from the substrate to the floating-gate through a thin tunneling oxide layer. A programming voltage pulse as high as 18-20 volts may be needed to induce tunneling. As charge builds up on the floating-gate, the electric field is reduced, decreasing electron flow. During programming, the select transistor is used to isolate the unselected EEPROM cells that are located on the same column. Erasing of the EEPROM cell is accomplished by applying a high programming voltage pulse to the control electrode of the floating-gate transistor.
An EEPROM is limited in the number of programming cycles it can experience before failure due to overstress. The overstress is caused by the high programming voltage pulses. The peak tunneling current has a direct bearing on EEPROM reliability and service life, and is related to the ramp-up rate of the programming voltage pulse. If the ramp-up rate of the programming voltage pulse is too high, the peak tunneling current may overstress the thin tunneling oxide layer, resulting in reduced reliability and shortened service life.
Typically, a charge pump, located on the same integrated circuit as the EEPROM array is used to generate the programming voltage pulses. The charge pump usually includes a plurality of series-connected pump stages that are driven by two nonoverlapping clock signals. The series-connected pump stages multiply the amplitude of the clock signals. The actual voltage obtained at the charge pump output terminal depends on the number of pump stages and on the charge transfer efficiency of the pump stages. In addition to providing a programming voltage pulse to the EEPROM cells, the charge pump may also be used to support decoding. Therefore, the charge pump is subjected to varying load conditions that may affect the ramp-up rate of the programming voltage pulse. Under heavy loading, the ramp-up rate may be relatively slow. If the load is suddenly reduced, the ramp-up rate may increase dramatically, overstressing the tunneling oxide layer of the floating-gate transistor.